Cathodes and emissive material therefor



2,871,196 Ice Patented Jan. 27, 1959 CATHODES AND EMISSIVE MATERIAL THEREFOR Dimitrios M. Speros, Willoughby, Ohio, assignor to General Electric Company, a corporation of New York No Drawing. Application April 29, 1957 Serial'No. 655,534

4 Claims. (Cl. 252-512) This invention relates to cathodes for electric discharge devices and more particularly to cathodes provided with an improved electron emissive material, to

methods of activating a cathode for emission of electrons, and to the emissive material itself. The invention is especially adapted for use in photographic flashtubes operating with cold cathodes.

Photographic flashtubes give a high intensity flash of very short duration and may be fired intermittently by means of a portable power pack as pictures are taken. They comprise a small diameter glass tube, filled with a rare gas such as xenon, the tube being provided at each end respectively with a cathode and an anode between which the electric arc discharge in the gas takes places. When used with portable photographic equipment, it is desirable to provide a flashtube operating at low voltages which increases the reliability of the unit particularly when batteries in the power pack become weak. Moreover, it is desirable t'o'operate the fiashtube with a cold cathode to simplify the firing circuit and to so design the cathode and emissive coating as to eliminate sputtering and deterioration of the "arc cent the cathode.

Therefore, the instant invention has among its ob-' jects to provide an emissive material for cathodes of electric discharge devices such as flashtubes which makes possible operation of the tube at low voltage and which also provides for efiective electron emission under cold cathode operating conditions.

- Another object is to provide a highly effective emissive material for discharge cathodes which utilizes barium aluminate having an excess of aluminum oxide together with some elementary barium bound into a com posite crystal matrix.

Another object of the invention is to provide a new and improved method of coating or activating a discharge cathode with emissive material utilizing a'barium aluminate system in which the emissive material is acti' vated by heat when in position on the cathode. Still another object is to provide a highly effective activated emissive material for discharge cathodes which allows latitude in choice of size and shape of the cathode.

It is well known that barium oxide itself (BaO) is an eifective emissive material for cathodes of electric discharge devices but this material has the disadvantage that it is subject to sputtering from the cathode to the walls of the discharge tube. In the case of photographic flashtubes, such emissive material is deposited on the Walls of the glass tube adjacent the cathodes with the resultant blackening of the tube thereby reducing the light output, and therefore the efiiciency of the flashtube. Barium oxide does have the advantage of a relatively high melting point which is in the neighborhood of l'900 C. so that this emissive material can withstand the high temperatures which occur at the spots on the cathodes where electron emission takes place and between which the electric discharge or flash takes place.

To reduce the undesirable sputtering characteristics of barium oxide, it has been proposed to add aluminum oxide (A1 0 to the barium oxide to form a barium aluminate. As aluminum oxide is added to increase its percentage in the compound, the melting point drops 01f rapidly and there is also a sharp reduction in the electron emission of the barium aluminate system. For example, if one mole of aluminum oxide is added to three moles of barium oxide to form the compound 3BaO-Al O the melting point of the compound drops to approximately 1750 C. If, however, the parts of barium oxide and aluminum oxide are made equal to form the compound BaO-Al O it is found that the melting point is comparable to the melting point of the original starting material barium oxide. Although the compound BaO-Al O has the very desirable high melting point, it is very low in electron emissivity and is unsuited for use as an activating material for cathodes of electric discharge devices such as flashtubes because of the high starting voltage required to flash the tube. If barium oxide (BaO) is added to increase the emissivity it will be found, as previously indicated, that the melting point of the compound drops rapidly. If the amount of barium oxide in the barium aluminate is further reduced to less than one mole ,a further reduction in emissivity takes place but the melting point remains ,at a satisfactory level about 1800" G. However, it has been found according to this invention that the emissivity of the barium aluminate may be greatly increased without an appreciable reduction in the melting point by converting part of the barium oxide in the barium aluminate to elementary barium. It is believed supporting electron emissive material; this increases the life of the flashtube and reduces end blackening adjaa highly eflfective emissive material.

that the barium is dispersed throughout the barium aluminate crystal, the result being that the crystal provides The emissivity of the aluminate system, including elementary barium, is in excess of the emissivity of barium oxide alone. Although the theoretical reason for this is not precisely known, it is believed that the increasedemissivity is due to the increased amount of elementary barium which may be bound in the crystal matrix of BaO-Al O as distinguished from the amount of barium which can be held by barium oxide (BaO) crystals. The emissive material thus described for activating cathodes may be prepared by mixing commercially available compounds of barium oxide-aluminum oxide (BaO-Al O with powdered aluminum and then activating the material by heating to convert part of the barium oxide to elementary barium. In manufacture of cathodes for electric discharge devices a mixture may be deposited on the .metal cathode and activated by heating in position.

iiThe activated emisisve material of the invention is characterized by the formula (xBaO.Al O ).yBa wherein there is an excess of aluminum oxide in the barium oxidealuminum oxide crystal to provide a compound having a high melting point. -x therefore must be equal to or less than 1. The lower limit of x is determined by the minimum quantity of barium oxide in the emissive material which will give satisfactory emission characteristics. x may range from 1 mole to as low as 0.1 mole; however, optimum results are obtained when x is in the range of 1 mole to 0.34 mole. The limits y of elementary barium are believed to be determined by the required number of emitting centers bonded in the barium aluminate crystal to give good emissivity.

v The upper limit of the elementary barium does not appear to be critical as some excess of elementary barium merely acts as a getter. An excess of elementary barium may be desirable for this reason; a large excess of barium,

occurs when this excess exceeds the moles of x'Ba'O by 2 to 2.6 times. Other getters may be used, for example, a titanium anode will act as agetter making excess elementary barium unnecessary. The lower limit of elementary barium ideallydependson the stoichiometric'yalue of one mole of elementary barium to be bound in each mole of BaO. Therefore y ideally approaches as the lowertlimit for optimum results; however, y may be as low as 05x and produce satisfactory results. :For'the reasons .given, it will be seen that y-may vary between 0.5x and 2.6x.

In preparing the emissive material, it is possible 'to use an available barium aluminate compound containing an excess of barium oxide as a starting material which has substantially the formula l.76BaO.Al Q Because this material is unstable in air it must be "stored and shipped in evacuated ampoul'es to protect it from deterioration by water vapor. This barium aluminate compound is first ground'to a powder and then mixed with aluminum powder which is preferably of a particle size about 0.003- mm. in properproportion as described below.

The amount of aluminum powder added to the above compound may vary between the limits of 2.7 to 7.3% maximum by weight of the mixture. However, aluminum powder in amounts between the limits of 2.7% and 5.8% by weight of the mixture gives optimum results. The

limits of 2.7% to 5.8% of aluminumpowder above stated correspond: to 0, 38 and 0.85 moles of aluminum respectivelyfor'each mole of 1.76BaO.Al O 'lhe chemi cal reaction occurring upon heating to between 600 C. to 660 -C. activates-the compound and is as follows in the two cases given:

If thefarno'u'nt of aluminum powder-is decreased substan'tia'lly below the limit given "an e'missionmaterial rich in barium oxide rather than rich in aluminum oxide results which, as previously mentioned, lowers themelting point. On the other hand the maximum weight of aluminum which may be utilized should not exceed theabove limit since this will deplete the barium oxide content of the cathode beyond the point of desirable emission and stability characteristics.

The electricdischa rge device or fiashtube is provided with conventional cold cathode constructions such as nickel grids, iron cups or tungsten coils. The emissive material is applied to the cathodes before activation. This may be accomplished by dipping'the cathode into a dry powder mixture, by dipping it into a slurry of the mixture with butyl acetate, by insertingparticles or pellets or the pressed material into or onto the cathode, orin'any other conventional method. Because of the good adhesion of the dry powder to tungsten coils, the dry method is probably easiest and works well fonportable flashtubes having tungsten c'oil "cathodes. For discharge lamps where a large quantityof emissive material is needed, the wet method is preferred but care should be taken to dry the cathodes thoroughly before the-heating operation hereinafter described or the emissive material may discolor and fiake off. An important advantage to the emission material according to the present invention is that a coating thickness hardly visible to the naked eye is sufficient.

After coating, the cathode is heated for activation of the emissive material. The cathodemay be heated to a temperature between 600 C. and 1200" C.;'however, about 600 C. appears to be a sufliciently high temperature. Activation at this comparatively low temperature makes it possible to construct the cathodes of low meltingpoint metals such as iron or nickel. -An' important advantage of this cathode is that no gases are evolved .during a'ctivation. Any'conventional method of heating may be,

4 applied, such as radiation, passing current through a cathode coil, conduction from a hot surface, or by low pressure are and glow discharges from the coil in argon. Activation may take place in a hydrogen atmosphere, argon or other inert atmosphere, or in a vacuum. Another advantage of the low temperature activation point is that it is possible to activate the coated cathode .by means of heat conducted to the cathode from the sealing fires used to seal the cathodes in the glass tube.

Cathodesprovided with an electron emissive-material h ave'been constructed as follows:

xample 1.-A coiled tungsten wire cathode having an emissive material with approximately the composition 1.43 (0.34BaO.Al O ')l.27Ba has been constructed by first grinding 94.2.grams of commercially available barium aluminate having the formula l.76BaO.Al O into a powder and mixing it with 5.8grams of aluminum powder (average particle size 0003 mm.). This mixture was thenpressed into pellets with'a pressure of 10 kilopounds per square inch and the pellets'recrushed in a mortar. Thclg'rinding, weighing and pressing was done inan airconditioned, low-humidity room, andthe material stored in a desiccator. The barium aluminate-aluminum mixture wasthenapplied to the cathode tungsten coils by dipping thecoils into the dry powder. The emissive material was thenactivated by heating the coated-coil to about 660 C. by means of the fires which seal the cathode to "the glass tube of the fiashtube.

Whilean emissive material prepared with the above startingmaterials issatisfactormbarium aluminate containing an excess of barium oxide. is unstable in air as previously mentioned, and may present some production This corresponds to.0..33Ba0-Al O -0.5Ba. This mixture was prepared as above and the dry mixture applied to the cathode. It was then activated by heating to atemperature of about 660 .C.

An emissive material of barium aluminate may be synthesizedand partially reduced-by aluminum .during the activation of thecathode .itselfin .a single step. Compounds consisting ofmixtures of barium. carbonate (BaCOg), .aluminum oxide (A1 0 and aluminum (Al) powder have been successfully reacted to produce upon being heated activated cathodes of the same compositions as the ones'described above.

This procedure, however, is not as susceptible to exact control as those already described. The reasons for this are that two solid state reactions must .now take place; (1) the reduction of the carbonate to the oxide at about 660 C. as represented by the equation 7 and (2) the partial reduction-of'the bariumox'ide to give Depending on diffusion factors, such as particle size, degree of pressing, rate of heating and the like, the less thermodynamically favorable Reaction 2 above may take place concurrently with Reaction 1 above thereby depleting the amount of aluminum present and necessitating the increase of activation temperature to approximately 1300" C. for the decomposition of unreacted barium carbonate.

Photographic flashtubes provided with cathodes having an emissive material have been constructed, tested, and compared with flashtubes heretofore manufactured but in which the cathodes comprised a nickel screen on which a barium-aluminum alloy had been vaporized. One standard type of photographic flashtube now commercially available and provided with such cathodes has a rated starting voltage of 305 volts with a standard deviation in the starting voltage of 20.3 volts. An allotment of 100 photographic flashtubes in which the cathodes were provided with emissive material comprising the barium-aluminate-barium compound according to the invention showed an average starting voltage of 286 volts with a standard deviation of 18.2 volts. The results show that flashtubes provided with cathodes constructed in accordance with the instant invention have a lower starting voltage than the identified standard flashtube' and this provides a more reliable flashtube for use with portable power packs used in photography. A sampling of twelve of the flashtubes were tested for 5000 flashes. Those provided with the barium aluminate cathodes showed little blackening on the ends whereas the standard tubes with the above-described nickel cathodes were blackened to an extent such that their light output was reduced 30 to 50%. This means that the barium aluminate emissive material on the cold cathodes is very 'eflective in eliminating sputtering and deterioration of the cathode under repeated flashing.

Another type of standard photographic flashtube currently available has a starting voltage of 182 volts with a standard deviation in starting voltage of 14.8 volts. Another allotment of 30 flashtubes was constructed in which the nickel screen cathode was replaced with tungsten coil cathodes having a barium aluminate emissive material of the composition according to the invention. The average starting voltage of this allotment of flashtubes was 168 volts with a standard deviation of volts. Again it is seen that by use of the instant invention, the starting voltage of the flashtube is reduced thereby increasing its reliability and effectiveness. Operation of the improved flashtubes up to 7000 flashes showed very little blackening of the flashtube whereas 6 the described standard lamps used as a basis of comparison showed that their light output was reduced b about 50% at 5000 flashes.

Still another type of flashtube was designed for repetitive. application and made with an emission material in accordance with this invention. The average life of these tubes before they began missing was 765,000 flashes; whereas, similar lamps with standard'cathodes do not exceed 250,000 flashes and are accompanied by early blackening. The barium aluminate cathodes showed very little blackening until near the end of their life.

The test results show that photographic flashtubes having cathodes provided with an activated emissive material composed of Al O and Ba bound in a BaAl O crystal matrix in the appropriate proportion range show very good maintenance in operation with little bulb blackening making the flashtubes particularly adaptable for use with portable photographic equipment.

While there have been described and illustrated specific embodiments of the invention, it will be obvious that various changes and modifications may be made therein without departing from the spirit of the invention which should be limited only by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A cathode comprising a conducting core and an emissive material applied thereto corresponding to the formula (xBaO-Al O )-yBa wherein x is between 0.1 and 1 and y is between 0.5x and 2.6x.

2. A cathode comprising a conducting core and an emissive material applied thereto having essentially the formula 0.33BaO-Al O -yBa wherein y is between 0.5 and 0.89.

3. An activated emissive material for a discharge cathode corresponding to the formula (xBaO-Al O -yBa wherein x is between 0.1 and 1 and y is between 0.5x and 2.6x.

4. An activated emissive material for a discharge cathode represented by the formula 0.33BaO-Al 0 -yBa wherein y is between 0.5 and 0.89.

References Cited in the file of this patent UNITED STATES PATENTS 1,921,066 Bedford Aug. 8, 1933 1,934,830 Spanner et al Nov. 14, 1933 1,946,603 Wedel Feb. 13, 1934 2,085,605 Ramsay June 29, 1937 2,142,331 Prescott Jan. 3, 1939 2,300,959 Pirani Nov. 3, 1942 

3. ACTIVATED EMISSIVE MATERIAL FOR A DISCHARGE CATHODE CORRESPONDING TO THE FORMULA (XBAO.AL2O3). YBA WHEREIN X IS BETWEEN 0.1 AND 1 AND IS BETWEEN 0.5X AND 2.6X. 